Didnt know you could top up by text.

Still shows 120 days on their site as you say.I top up 43 days ago,and still have 77 days left.

I use Voipdiscount, same company, just different prices and countries. Today I got this email.

Dear

We would like to remind you that your VoipStunt credit will expire on 2006-06-27.
Please use your credit before it expires and/or buy new credit. If you buy new credit before your old credit expires any old credit left in your account will also remain valid.

Kind regards,
VoipStunt Customer Services
www.voipstunt.com

So assuming that voipdiscount operates in a similar way, you won't actually lose your credit.
Although having to top up every 60 days is a bit much, you need to look at how much you use their service to justify whether it's worth staying there or just moving your custom